Optical networks that, at the physical layer, include optical fiber transmission media and electrical domain switches, are well known in the art. A substantial proportion of today's data traffic traverses synchronous optical network (SONET), and synchronous digital hierarchy (SDH) standard networks, as well as converged SONET-SDH networks. These and other networks besides, provide an important failure recovery mechanism known as protection switching.
In accordance with earlier versions of the optical network standards (such as the current SONET ring standard issued by Telcordia GR-1230-CORE), each optical fiber span that interconnected adjacent network elements (NEs) was paired with a dedicated protection fiber span. In this way, when a failure condition is detected on a working channel through the fiber spans, automatic protection switching (APS) information (transmitted in an overhead of a frame for transporting the data) was used to switch the traffic to a protection channel defined over the protection fiber spans associated with the failed working fiber spans of the working channel. The costs of providing and maintaining dedicated protection fiber have led to two major improvements to protection switching schemes.
A first major improvement involved permitting use of the protection fiber spans for network traffic until a working path failure was detected by introducing an unprotected class of traffic, usually referred to as extra traffic. If a working fiber span associated with a protection fiber span failed, the extra traffic was “dropped” and the protected traffic on the working fiber span was sent over the protection fiber span. Extra traffic was therefore generally unreliable. So while the unused bandwidth of the otherwise unused protection fiber strand can be used, the value of this bandwidth is low.
The second major improvement to protection switching schemes involved permitting multiple working connections to ‘reserve’ respective chains of resources through a network, so that in the event of failure of one of the working paths, the failing connection can seize network resources, and establish a protection connection. Such protection schemes are known as 1:N protection schemes or shared protection schemes. A 1:N protection scheme that permits upto N working connections to share any protection resource, has been implemented on linear SONET/SDH network configurations.
In linear SONET/SDH network configurations, a NE at a downstream end of a channel that detects a failure may issue a request for protection switching by the NE at the upstream end. If the condition of the protection channel at the upstream NE indicates a higher priority user/request, the request from the downstream end is dropped and the other request at the higher priority is forwarded to the downstream NE, which is then obliged to cede the protection channel. Similarly, if a lower priority request for a channel is allowed before a higher priority request for the channel is received, the use of the channel is given to the higher priority requester, and the other (lower priority) channel is forced to cede the channel. Thus concurrent failures of multiple working channels are handled using a hierarchy of pre-emption values.
Generally the pre-emption priority hierarchy includes conditions for requesting a protection switch/occupying the protection bandwidth, including; a signal fail on the working channel, a signal degrade on the working channel, a wait to restore period after a signal fail/degrade on the working channel, a manual switch requested by network management, and a forced switch requested by network management. The working channel may further be associated with a grade of service that is used in the hierarchy. This pre-emption priority hierarchy is used to ensure that a protection access policy is followed. A protection access policy may include rules such as, for example: that a signal degrade on one channel does not pre-empt a signal failure on another, as a signal degrade has less impact on traffic than a signal failure; that a manual switch can be pre-empted by a signal degrade, so that a manual switch does not interrupt any traffic; that a forced switch cannot be pre-empted by any automatic protection condition; etc.
These two improvements are not mutually exclusive. Extra traffic is carried on current linear and ring SONET/SDH networks. It can be said that extra traffic represents a priority level equal to a “no priority reversion” message used to indicate that traffic is being returned to the working channel (i.e. a lowest priority level). It has further been identified that unprotected traffic is a desired class of service in its own right. More specifically, non-pre-emptable unprotected traffic (NUT) is a class of service that, as its name suggests, is assigned to a channel that cannot be used as protection, but is not, itself, protected. NUT ranks between protected traffic, and extra traffic when it comes to reliability in the sense that it is dropped if a loss of signal occurs; but it cannot be pre-empted by a working channel because the working channel has failed. Networks that provide NUT effectively take the channels devoted to NUT out of use for protection purposes. This designation of links as one of NUT, working and protection does not provide for desired flexibility.
What is desired is a method of handling protection switch requests that provides more efficient use of bandwidth, and specifically provides for the enforcement of a protection access policy that enables grades of service of unprotected traffic over protection channels, and other paths through an optical network.